Polyesters, including poly(ethylene terephthalate) (PET), are most commonly made by condensation polymerization starting from one or more diols and one or more diacids or diesters. In order to achieve high enough polyester molecular weights in reasonable amounts of time, one or more catalysts is typically added to the polymerization process. Such catalysts in general include those based for example on titanium, antimony, tin or other elements. Some catalysts contain more than one metal, for example Sb is often present with Mn or Zn. In recent times, when making PET, the polymerization catalyst almost always included an antimony compound, such as antimony oxide or sodium antimonate. Relatively recently in some localities such as Japan, especially for PET meant for use in bottles, the toxicity of antimony has become a concern, and other catalysts such as those based on germanium have been utilized for this use.
When used for certain shaped parts and/or when highly reinforced it is sometimes desirable to toughen polyester compositions. PET and other polyester compositions have been most commonly toughened by adding a rubber or rubber-like polymer to the PET, and thoroughly mixing in this rubber so that it is dispersed as small particles within a PET continuous matrix. Usually this “polymeric toughener” has attached (bonded) to it functional groups such as epoxy or carboxylic anhydride, which are usually thought of as being able to react with functional groups on the PET such as carboxyl or hydroxyl end groups, see for instance U.S. Pat. No. 4,753,980. Herein this type of toughener is referred to as a “functionalized polymeric toughener” (FPT).
These toughened PET (TPET) compositions are typically used for forming shaped parts by melt forming processes such as extrusion, injection molding, or blow molding. During manufacture of the TPET and subsequent melt forming operation(s) the TPET is heated above the melting point of the PET, typically about 265-290° C. This heating period is typically desirably limited to that amount of time required to melt form the TPET to prevent thermal degradation, often 1-3 minutes, but sometimes it may be undesirably extended for instance to 5-10 minutes or even longer, because of a malfunction of the melt forming machine or process, or other unscheduled interruption. When this occurs such TPET compositions usually suffer from a decrease in melt viscosity and/or toughness in the resulting product. Therefore methods to improve the heat stability of TPET compositions are desired.